Gmsh: exporting old msh format

This post is mainly aimed at my colleagues (and students) in the LMCG research group at LITPEG. After many people going to me, asking about a bug of p3matpac preprocessor, I decide to write it.

After the Gmsh 4.0.0 release, Christophe Geuzaine and Jean-François Remacle set the new msh version (msh4) as default msh format (I guess... ). After that, new students and colleagues have faced with a following error:

>>> g = gridprocessing.load_gmsh('mesh_test.msh')
Unknown mesh format: 4.1 0 8

There are 3 ways to fix this: GUI, CLI, and script ¹.

GUI mode

You need to have the following steps:

1) Press Ctrl+E shortcut to open the export window:

2) Select the Version 2 ASCII and OK.

Done!!

CLI mode

Using the command line interface all you need is run this:

gmsh mesh_test.geo -2 -format msh2

Script

Just put this Mesh.MshFileVersion = 2.2; into *.geo script file.

Other way

Using this mode, gmsh will always save in msh format 2.2. All you need is add this lines:

Mesh.Format = 1; // msh output format
Mesh.MshFileVersion = 2.2; // Version of the MSH file format to use

Into .gmshrc file, by this command:

echo "Mesh.Format = 1; // msh output format
Mesh.MshFileVersion = 2.2; // Version of the MSH file format to use" >> ~/.gmshrc